JP 2001-8441 A discloses related art of a voltage conversion device of this type. A voltage conversion device of Patent Document 1 is a DC-DC converter which converts direct current power from a secondary battery (direct current power supply) to direct current power of a different voltage value utilizing a switching operation of a transistor and outputs the converted power, and the cycle of the pulse applied to the transistor is changed based on a temperature around the DC-DC converter.
In a DC-DC converter, in some cases, a capacitor (filtering capacitor) is provided at the input side in parallel with the direct current power supply (secondary battery). During a switching operation of the transistor of the DC-DC converter, a ripple component is generated in current flowing in a reactor. When the filtering capacitor is provided in parallel with the direct current power supply, the current flowing through the reactor becomes a current in which the current of the filtering capacitor (ripple component) is superposed on a current of the direct current power supply (direct current component). With such a configuration, variation in current of the direct current power supply can be suppressed.
In addition, in such a DC-DC converter, heat is generated in the switching element during the switching operation of the switching element, and the amount of generated heat in the switching element is increased as the switching frequency is increased. In order to prevent overheating of the switching element (transistor), the switching frequency (carrier frequency) of the switching element is changed. For example, when the temperature of the switching element is higher than a predetermined temperature, the switching frequency of the switching element is reduced so that temperature increase of the switching element is suppressed.
However, when the switching frequency of the switching element is reduced, although the temperature increase of the switching element is suppressed, the ripple component of the current flowing through a reactor is increased, and consequently the extent of temperature increase due to the ripple current of the reactor is increased, resulting in a higher tendency to overheat. Furthermore, when the filtering capacitor is provided in parallel with the direct current power supply, if the switching frequency of the switching element is reduced, the ripple current flowing through the filtering capacitor is also increased, and consequently the extent of temperature increase due to the ripple current of the filtering capacitor is increased, resulting in a higher tendency to overheat. As described, in a DC-DC converter, the temperature increase characteristic of the switching element and the temperature increase characteristic of a reactor and a filtering capacitor have opposite characteristics with respect to change of the switching frequency. Because of this, although the overheating of the switching element can be suppressed by changing the switching frequency of the switching element according to the temperature of the switching element, it is not possible to prevent the overheating of the filtering capacitor or the reactor.